1. Field of the Invention
The present invention relates to an electrophotographic sensitive member having a photoconductive layer of amorphous silicon.
2. Description of the Prior Art
Electrophotographic sensitive members of various types have already been proposed. Among other things, attention has been focused on the application to such sensitive members of amorphous silicon (hereinafter abbreviated as "a-Si") which is produced by the processes such as glow discharge and sputtering and to which research and development work has been directed in the field of semicounductors in recent years. This is attributable to the fact that photosensitive members of a-Si are much superior to those of selenium or CdS with respect to heat resistance, abrasion resistance and photosensitive characteristics, and also from the viewpoint of environmental pollution. A photosensitive member comprising a photoconductive layer of a-Si is proposed in U.K. Patent Application No. 2,013,725 (published Aug. 15, 1979).
Having actually conducted research on the application of a-Si to electrophotographic sensitive members, the inventors have found that the sensitive member of a-Si, unlike the conventional sensitive members, has the ideal characteristics of causing no pollution and being outstanding in heat resistance, surface hardness, abrasion resistance, etc. However, the inventors have encountered the problem that when the usual glow discharge or sputtering process is resorted to for the preparation of a-Si, the resulting photoconductive layer has a dark volume resistivity of as low as up to about 10.sup.10 ohm-cm, is not chargeable even to the lowest of the surface potential levels required for forming images by the Carlson process and is therefore unusable as it is for the sensitive member suited to the Carlson process which comprises the steps of charging, exposure to the original image, development, transfer, cleaning and charge erasing.
As reported by W. E. Spear and P. G. Le Comber in "Electronic Properties of Substitutionally Doped Amorphous Si and Ge," Philosophical Magazine, Vol. 33, No. 6, pp. 935-949, 1976 in the field of semiconductors, a-Si, when in a pure form free from impurities, acts usually as an N-type semiconductor with its structural defect providing a doner level, and when having incorporated therein a Group Vb impurity element of the Periodic Table, usually phosphorus (P), serves as an intensified N-type semiconductor, whereas a-Si serves as a P-type semiconductor when containing a group IIIa impurity element, usually boron (B), added thereto. The dark resistivity of a-Si varies in accordance with the impurity content. In fact, the report shows that addition of B.sub.2 H.sub.6 and SiH.sub.4 the material of a-Si, in an amount of 10.sup.-4 to 10.sup.-5 in mole ratio (200 to 20 ppm) results in an increased dark resistivity of about 10.sup.11 ohm-cm. However, use of a larger amount of boron entails a marked decrease in dark resistivity, because the effect achieved by the addition of impurities to a-Si is generally lower than is the case with crystalline Si and is much lower when the a-Si is used for electrophotographic photoconductive layers. According to the above-mentioned U.K. Patent Application No. 2013725, the preferred amount of a Group IIIa impurity element to be added is 10.sup.-3 to 10.sup.-6 atomic % (corresponding to 5.times.10.sup.-6 to 5.times.10.sup.-9 in B.sub.2 H.sub.6 /SiH.sub.4 mole ratio or 0.01 to 10 ppm) which is exceedingly lower than the amount proposed by W. E. Spear et al. in the field of semiconductors. It is therefore thought that with a-Si for use as an electrophotographic photoconductive material, the addition of the Group IIIa impurity element is not useful for significantly controlling the electroconductivity (dark resistivity) and fails to give a greatly increased dark resistivity. In view of the situation described above, it is desired to provide sensitive members having a photoconductive layer of a-Si which is adapted to have incorporated therein a very small to large amount of impurities and the electroconductivity of which is easily controllable over a wide range.
It is also strongly desired to develop an a-Si photoconductive layer having a dark resistivity of at least about 10.sup.-- ohm-cm as required by the electrophotographic image-forming process. To fulfill this requirement, Published Unexamined Japanese Patent Application No. SHO 54-145539 proposes to incorporate 0.1 to 30 atomic % of oxygen into a-Si and thereby improve the dark resistivity. When the inventors have actually prepared photoconductive layeers from a-Si containing at least 0.1 atomic % of oxygen and checked the layers for general electrophotographic characteristics, the layers were found to have an improved dark resistivity wich was well above the value required for the electrophotographic process. However, the photosensitive characteristics greatly reduced with an increase in the oxygen content and that even when having the lowest oxygen content of 0.1 atomic %, the layer was quite inferior to those of conventional photosensitive members in photosensitivity in the range of visible light.
Furthermore, sensitive members comprising an a-Si photoconductive layer formed directly over a conductive substrate and having an image-forming surface are low in production stability and reproducibility, and the electrophotographic characteristics of such sensitive members, especially the charge accepting capability and dark volume resistivity thereof, considerably vary from member to member even when they are produced by the same method. Thus the sensitive members in which the a-Si photoconductive layer is formed directly over a conductive substrate still remain to be substantially improved in manufacturing stability and reproducibility for actualy use.